This invention relates to apparatus and method for canceling noise and more particularly to apparatus and method for canceling amplitude and phase modulation noise in amplifiers.
As is known in the art, amplifiers are used in a wide variety of applications. In a radar system application, an amplifier, such as a travelling wave tube (TWT) amplifier, is used to amplify a local oscillator signal having the carrier frequency of the radar signal to be transmitted. In response to a radar system trigger signal, the TWT is activated to thereby produce at its output the high power radar signal.
As is also known in the art, because of thermal noise within the TWT amplifier and the effects of ripple in the power supply used to power the TWT amplifier, the amplified radio frequency signal will contain, in addition to the carrier frequency, other unwanted frequency components. These unwanted frequency components, i.e., amplitude modulation (AM) noise and phase modulation (PM) noise, can detract from the performance of the radar system.
One method suggested to reduce this amplitude modulation noise is to provide an electronically controllable attenuator at the input to the TWT amplifier and to feed a small portion of the power from the output as the control signal for the attenuator. Such an arrangement, however, prevents the TWT from operating at maximum power thereby reducing the detectable target range of the radar.